Abstract In this paper, the fracture behavior and acoustic emission (AE) characteristics of reinforced concrete under mixed mode I-II load conditions are studied using three-point bending beams with a straight offset notch. Different offset ratios, different crack-depth ratios, and different sizes of reinforced concrete specimens and a set of concrete specimens as a control group were explored. The critical loads, the crack mouth opening displacements, the final failure angles, and the crack trajectory was recorded to analyze the fracture behavior of specimens under different factors. The 3D crack source locations by AE technique was adopted to locate the damage during the fracture process, which was in a good agreement with the crack propagation path obtained through the fracture test. A criterion, based on the simultaneous appearance of the rise of RA (rise time/peak amplitude), the decline of AF (counts/duration time), and Ib-value (ratio of weak events to strong events), was proposed to identify the three critical conditions in the mixed mode I-II fracture process, which include the initial crack at the notch tip, crack at the bottom of mid-span, and the peak load. Then, the fracture process was divided into four stages: initiation of micro-crack, stable crack propagation in a single path, stable crack propagation in double paths, and unstable crack propagation. The AE hits can accurately distinguish the moments of crack at mid-span and peak load. The modified AE parameters analysis method was utilized to classify the cracking mode at different stages of damage and the results showed that the shear cracks followed the tensile cracks, the proportion of AF to RA for common specimens in this study can be set to 80: 1, while that of small size specimen with a span length of 600 mm can be set to 40: 1. The results can provide a theoretical basis for the establishment of remote monitoring and early warning systems for large structures.

中文翻译：

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混合模态Ⅰ-Ⅱ荷载条件下钢筋混凝土的断裂行为及声发射特性

摘要 在本文中，使用具有直偏置缺口的三点弯曲梁研究了混合模式I-II 荷载条件下钢筋混凝土的断裂行为和声发射(AE) 特性。探索了不同偏移比、不同裂缝深度比和不同尺寸的钢筋混凝土试件和一组混凝土试件作为对照组。记录临界载荷、裂纹口张开位移、最终破坏角和裂纹轨迹，以分析试样在不同因素下的断裂行为。采用AE技术的3D裂纹源位置定位断裂过程中的损伤，与断裂试验得到的裂纹扩展路径吻合较好。一个标准，基于同时出现的 RA 上升（上升时间/峰值幅度）、AF 下降（计数/持续时间）和 Ib 值（弱事件与强事件的比率），提出了确定三个关键I-II 型混合断裂过程中的条件，包括缺口尖端的初始裂纹、跨中底部的裂纹和峰值载荷。然后将断裂过程分为微裂纹萌生、裂纹单路径稳定扩展、双路径裂纹稳定扩展和裂纹不稳定扩展四个阶段。AE 命中可以准确区分跨中和峰值载荷下的裂纹力矩。利用修正的声发射参数分析方法对不同损伤阶段的开裂模式进行分类，结果表明剪切裂纹继拉伸裂纹之后，